1. Field of the Invention
The present invention relates to a method and apparatus for transferring a thin plate for use in transferring a thin plate such as a wafer for manufacturing a semiconductor, a substrate for a magnetic disc, and the like.
2. Description of the Related Art
A wafer for manufacturing a semiconductor comprises a thin plate of silicon and the like. Semiconductor devices and electronic circuits are made on the surface of a wafer with photo engraving technology and various types of microfabrication technology. In processing such a wafer, it is necessary to securely transfer the wafer with holding only the vicinity of the periphery thereof, in order to prevent a defect and quality deficiency from occurring due to dust and the like adhering to the surface thereof, and to increase an effective chip area.
Recently, for the purpose of realizing finer-line processes of semiconductor devices and improving productivity thereof, a wafer increasing in size from conventional 200 mm to 300 mm is used, and the semiconductor devices and the electronic circuits made on the wafer also become denser. In accordance with increase in size of the thin plate like this, it becomes more and more important to securely transfer the thin plate in a clean state while holding only the periphery of the thin plate.
Correct and efficient transfer of a wafer is effective at reducing cost for manufacturing and testing, by means of increasing the number of processed wafers per one semiconductor manufacturing equipment. In a case of a measuring device which takes about one minute for measurement per one wafer, for example, it is necessary to mount the wafer within twenty seconds.
FIG. 9 shows a flatness measuring device for a wafer 100. The device is provided with a transfer arm 102 for transferring the wafer 100 to a measuring unit 101. The transfer arm 102 attached to a main body of the device is rotatable and movable in a horizontal direction. The transfer arm 102 comprises a first arm 103 with a claw for grasping a lower edge of the wafer 100 from beneath, and second and third arms 104 and 105 with claws for grasping an upper edge of the wafer 100 from two directions above the wafer 100. The measuring unit 101 has three holder claws 106, 107, and 108 for holding three spots in the outer edge of the wafer 100. An arrow in FIG. 9 shows a movable direction of the transfer arm 102.
The first to third arms 103 to 105 of the transfer arm 102 holds the wafer 100 and transfer it to the measuring unit 101. Then, the three holder claws 106 to 108 of the measuring unit 101 hold the wafer 100 on the measuring unit 101.
A transfer method will be hereinafter described in detail. After the transfer arm 102 moved by a holder unit 111 is brought near to the measuring unit 101 until an edge sensor attached on the edge of each arm 103 to 105 is actuated, an actuator for controlling deviating positions of the arms 103 to 105 is driven by signals from the sensor. The actuator conducts feedback control of the position of the arms so that the edge of the wafer 100 is positioned in the middle of the predetermined groove of the claw. Then, the whole wafer 100 is lifted by actuating a cam, by which the three claws on the edges of the arms are moved in a radius direction of the wafer. Then, a movable edge claw of the measuring unit 101 moves toward the wafer to receive the thin plate, wafer 100 with the claws 106 to 108 of the main body. After an edge detection sensor and a deviating position detection sensor confirm that the wafer is mounted on the measuring unit 101 by means of opening claws of three arms with the cam, the transfer arm 102 moves leftward and the transfer is completed.
To dismount the wafer 100, a transfer arm 110 shown in a lower portion of FIG. 9 is operated in a similar way with the rotation of a rotational shaft 109. To pass the wafer 100, both of the transfer arm for mounting 102 and the transfer arm for dismounting 110 are used.
In the conventional transfer method described above, however, a hand section passes above the wafer when the wafer is mounted on and dismounted from the measuring unit 101. Thus, there is a problem that the dust and the like falling down from the hand section tends to adhere to the wafer which is required to be clean.
In the flatness measuring device, three sensors are provided on the edges of the placement arms, and the positions thereof are sent back to conduct a feedback control. Thus, a control circuit which requires delicate adjustment is necessary for the purpose. Therefore, there are problems that a large amount of cost is necessary to adjust the assembly of the device, and the maintenance thereof is difficult.
The edge claws on the main body side are made of plastic to reduce dust, and plastic portions must be exchanged almost once a month. It is difficult to make an identical part with precision to less than 0.1 mm, due to the characteristics of the plastic. The exchanged plastic portions have to be attached on proper positions, so that the wafer does not eccentrically deviate with respect to the measuring unit. Thus, there is a problem that man-hour for adjustment increases.
In this way, the adjustment of the transfer unit including the actuator is troublesome in the conventional transfer unit. The conventional transfer unit also has the high risk of adhesion of dust on the top surface of the wafer. Furthermore, there is a problem that it is difficult to attach and adjust the wafer holder claws in exchanging.